This invention relates to a three-phase electrical transformer and a method for manufacturing thereof.
A transformer is a known electrical device widely used for transferring energy of an alternating current in the primary winding to that in one or more secondary windings. It typically contains two or more electrical circuits comprising primary and secondary windings, each made of a multi-turn coil of electrical conductors with one or more magnetic cores coupling the coils by transferring a magnetic flux therebetween.
Presently known three-phase transformers usually utilize E+1 magnetic cores in a flat structure. Such a transformer includes several interconnected magnetic cores located in one plane. U.S. Pat. Nos. 4,893,400 and 5,398,402 disclose transformers having a magnetic core made of an amorphous metal strip wound into a core over a mandrel, with one leg of the resulting core being subsequently cut off and with forming the metal into a rectangular shape. This transformer is manufactured in the following manner. A piece of rectangular steel is wrapped around the outer periphery of the amorphous metal core. The amorphous metal is then annealed, and the core is encapsulated in a resinous coating, except the cut leg. This allows the opening of the cut leg. The layers of amorphous alloy strips of the two edges are oriented so that the edges define top and bottom surfaces, each surface having a discontinuity defining a distributed gap portion extending from the top surface to the bottom surface. The coils are placed over two long legs and the cut leg is closed. The joint is then sealed.
According to U.S. ""400, the sealing is made with glass cloth and an ultraviolet-curable resin to provide the structure by the xe2x80x9cfit and curexe2x80x9d method. This method is costly and labor-intensive. The transformers having amorphous metal cores manufactured according to this method cannot be repaired without causing damage to the core.
According to U.S. ""402, the sealing is made with a porous material such as woven cotton cloth or paper. The porous material is folded over the joint and secured into position. An additional piece of porous material is placed through the window of the core, wrapped around the core and secured there. Electrical grade steel is disposed around the transformer core and is closed around the core joint and tack-welded. This structure allows the cut leg to be opened to permit replacement of a defective coil. The operation, however, is time-consuming and labor-intensive.
U.S. Pat. No. 5,441,783 discloses a technique of the kind specified, wherein a coating used to impregnate the core joint is a porous material with a viscosity greater than about 100,000 cps and a bonding material with a viscosity of at least about 100,000 cps. The porous material comprises strands of fiber, and the bonding material is thixotropic epoxy. Although the coated cores have good magnetic properties, their manufacture requires costly and complex operational steps. Moreover, the method of repairing these cores is labor-intensive.
Another common disadvantage of the transformers manufactured according to the techniques disclosed in the above patents is that annealed amorphous metals become extremely brittle, and thus break under mechanical stress, for example, during the stage of closing the core joint.
In the transformers of the above kind, a planar core structure is used. U.S. Pat. No. 4,639,705 discloses a transformer structure of another kind, having a spatial magnetic core system. This structure has advantages over the planar xe2x80x9cE+1xe2x80x9d structure, such as the reduced quantity of required magnetic materials (by about 20-30%), reduced volume of the transformer, reduced core losses (by about 20-30%), and balanced currents in the three phases of the primary windings. However, to manufacture a transformer in accordance with the technique disclosed in U.S. ""705, complex production technology as well as a complex repair technology, are required.
It is accordingly a need in the art to facilitate the manufacture and maintenance of a three-phase transformer, by providing a novel electrical transformer structure and a method of its manufacturing.
It is a major feature of the present invention to provide such a transformer that has higher efficiency and smaller magnetic core, and that uses lower quantities of materials per unit electrical power and/or has better maintainability, as compared to those of the conventional transformers of this kind.
The main idea of the present invention consists of constructing a three-phase transformer having a spatial symmetrical structure of a magnetic circuit. The magnetic circuit comprises two spaced-apart, parallel plate-like elements, and three spaced-apart parallel column-like elementary circuits, which are substantially perpendicular to the plates and are enclosed therebetween forming a mutually symmetrical structure.
There is thus provided according to one aspect of the present invention a three-phase transformer comprising a magnetic circuit and three coil blocks, wherein the magnetic circuit comprises:
two spaced-apart, parallel, plate-like elements; and
three spaced-apart, parallel column-like elementary circuits, each column carrying the corresponding one of said three coil blocks and serving for the corresponding one of the three phases, wherein the columns are substantially perpendicular to the plate-like elements and are enclosed therebetween such as to form a spatial symmetrical structure about a central axis of the transformer.
Preferably, each element of the magnetic circuit (i.e., plates and columns) is formed of an amorphous strip (e.g., ribbons of a soft ferromagnetic amorphous alloy) or a silicon steel strip. The plate-like element may be of a substantially triangular shape with rounded edges, or of a circular shape that simplifies the technological process of the manufacture of the plate-like element. The plate-like element may be a toroid.
Each of the column-like elementary circuits may be a toroid or several axially mounted toroids, each having a radial slot filled with an insulating material. Alternatively, each of the elementary circuits may be manufactured from a plurality of vertically aligned strips or ribbon pieces, in which case the cross section of the column is a polygon or a circle. The ribbon pieces are attached to each other, in such a manner that each ribbon piece is in a planar state and is oriented along the column.
The elementary circuits are spaced from each other and from the plate-like elements by insulating spacers. All the spacers may be formed of plastic with filler of a magnetic powder with the concentration of 20-50%.
Each of the toroids may be made of a set of amorphous strips having different widths. The alternation of the strips of different widths extends along the vertical axis of the toroid, and the strips of the adjacent layers are displaced from each other along the vertical axis in such a manner that the strips of one layer overlap the butts of the strips of the adjacent layer.
The working surfaces of the toroidal plates can be formed with annular concentric recesses, the butt-end surfaces of the vertical elements (columns) being formed with corresponding projections to be received by the recesses. The contacting surfaces of the recesses and projections should be coated with insulating materials.
The advantages of the present invention consist of the following. The provision of the plate-like elements of a triangular shape with rounded corners allows for effectively transferring the magnetic flux between the three column-like elementary circuits enclosed between the plates. The provision of the column-like elementary circuits formed by one or more toroids produced by wounding the amorphous strips, enables to obtain a desired height of the column irrespectively of the limited width of the strip. Moreover, the stacked structure of the column formed of several toroids provides good conductivity of the magnetic flux (low reluctance) along the column, while presenting high impedance to eddy currents. By forming the elementary circuit (column) with a radial slot, the eddy currents could be even more reduced. Actually, the introduction of the radial slot results in the induction of high voltage equivalent to that in one ribbon turn. Additionally, such a modular structure of the entire transformer simplifies its assembling and dismantling, thereby allowing the easy manufacture and maintenance of the transformer. Thus, by appropriately selecting the dimensions of the transformer""s elements (e.g., the diameter of each column-like element and each of the plate-like elements), the desired properties of the transformer can be achieved.
According to another aspect of the present invention, there is provided a method for manufacturing a three-phase transformer, the method comprising the steps of:
(i) producing two substantially plate-like elements of a magnetic circuit of the transformer from materials having soft ferromagnetic properties;
(ii) producing three column-like elementary circuits of said magnetic circuit from materials having soft-ferromagnetic properties;
(iii) mounting a coil block on each of the column-like elementary circuits to form the corresponding one of the three phases of the transformer,
(iv) mounting the column-like elementary circuits between the plate-like elements in a spaced-apart parallel relationship of the elementary circuits, such as to form a spatial symmetrical structure about a central axis of the transformer.